Cellulosic fibers obtained from a solution of directly dissolved cellulose have been classified by the International Bureau for Standardization in Brussels (BISFA) as lyocell fibers. In that case the dissolving of the cellulose and the processing of the cellulose spinning solution take place without chemical derivatization of the cellulose. The fibers can be utilized in one case as staple fibers with subsequent spinning into yarns, or can be utilized directly as continuous filaments in an operation for producing sheetlike textile structures by weaving, knitting, webforming, etc.
The term “shaped lyocell articles” used hereinafter encompasses fibers, filaments, nonwovens, films, and foams based on lyocell cellulose.
The term “flame retardant” refers to an agent which delays, or under normal conditions prevents, the burning of a product to which it has been applied or into which it has been Incorporated. This quality is referred to below as “low flammability”.
In order to assess the combustibility of samples, an index used in practice, besides the assessment of burning in a fire chamber, is the Limiting Oxygen Index (LOI), this being the minimum oxygen concentration in % at which a material still just burns.
Commercially available lyocell fibers of low flammability are very largely characterized by retrospective treatment of the fibers with coatings containing diverse substances, which are of limited durability and effect. Especially when the textiles are required to undergo wet treatments (laundering with different detergents, especially at elevated temperatures) and/or as a result of migration of the chemicals from the treated fibers, these retrospectively applied coatings come up against their limits. Furthermore, these coatings adversely impact the tactile qualities and water vapor transport capacity of the cellulosic fibers.
Within the field of endowing cellulosic fibers, produced by the viscose process, with flame retardancy, a solution is provided in U.S. Pat. No. 4,220,472, by means of a specific, phosphorus-containing flame retardant (dithiophosphoric anhydride), which is incorporated during fiber production. This solution, as well as other attempted solutions, do not produce a satisfactory outcome in the case of the lyocell process, particularly the industrial NMMO process implemented in the art, since the flame retardant does not withstand the operation of producing the lyocell fibers.
WO 2003040460 and DE 10038100 disclose various methods for imparting flame retardancy to cellulosic fibers, the finished fiber being subjected to treatment with cyanuric chlorine derivatives. A costly and inconvenient treatment procedure, associated with energy-related and environmental disadvantages; the presence of chlorine, which is critical in the event of fire, in the flame retardant; and a relatively low level of flame retardancy (LOI not more than 25) are disadvantages of such a procedure.
The redispersible dispersion-based powders of various copolymers, described in DE 4306808, are also applied to a cellulosic fiber which has already been completed, this method being more suited to fiber composites than to filaments or fibers which can be processed as textiles.
A feature common to all of these cited patents and the solutions they present is that only coating of the fiber surface with the flame retardant is realized. This coating is usually thin, and so does not result in a substantial and (wash-)permanent flame retardancy.
WO 1994026962 claims the impregnation of a freshly spun lyocell fiber, after washing and before drying, with a phosphorus-based flame retardant, followed by a fixing process. The treatment must be regarded as costly and inconvenient, and there will be a deterioration in textile qualities, such as the hand, of such fibers. Moreover, the compounds presented are not wash-resistant.
WO2011045673 describes flame-retardant lyocell fibers with incorporated inorganic flame retardants, such as kaolin or talc. These flame retardants are active only in very high proportions, and have deleterious effects on the physical textile qualities of the fibers. Consequently these fibers can be employed only in mattresses and upholstered furniture.
Other established phosphorus-containing flame retardants used in the production of viscose fibers, such as EXOLIT®(SANDOFLAM®), in WO2011026159, for example, do not withstand the thermal stress of the preparation of solution in a lyocell process, and are consequently not a suitable flame retardant system for lyocell processes.
Up to the present there has been no disclosure of a satisfactory solution which describes the production of shaped, low-flammability lyocell articles in a continuous production operation without substantial complication of the operation of the lyocell process, and which affords fibers, filaments, and nonwovens having sufficiently textile character. A great advantage here, relative to the viscose process, for example, is that in the lyocell process even a high fraction of additives in the fiber guarantee good mechanical properties, as are necessary for processing of the fibers on standard machinery in spinning, weaving, and finishing and also in the use of the sheetlike textiles.